If sunlight damages skin, why did evolution not give us better protection against it?

The second half is that we do have protection, and it is quite good — you are simply not counting it.

Melanin is a highly effective adaptation. Populations with long ancestral histories in high-sun regions have deeply pigmented skin that substantially reduces damage. That is exactly the defence you were expecting to find.

What has changed is not the biology but the situation:

• People moved. Skin pigmentation is adapted to a specific latitude and light environment, and modern populations frequently live nowhere near where their ancestors did.
• Behaviour changed. Long indoor periods followed by intense intermittent exposure — a week of holiday sun — is a pattern the system never encountered. Gradual continuous exposure builds tolerance; intermittent burning does not.
• Lifespans doubled. The damage now has decades to express itself in people who are still alive to experience it.

So the honest summary is a well-adapted system in an environment it was not adapted to.

The error is in the second option and it is the most common misconception about evolution: it does not optimise for health or longevity. It optimises for reproduction.

The damage you are describing accumulates over decades and its most serious consequences appear predominantly later in life — largely after the years in which our ancestors were having and raising children. A trait that harms you at sixty exerts very weak selective pressure, because your genes have already been passed on. Selection is close to blind to anything happening after reproduction is finished.

That single point resolves most of the puzzle. It is the same reason many age-related conditions exist at all: not because they are harmless, but because they are invisible to the process that shapes us.

There is also a genuine trade-off, which is worth knowing because it explains why pigmentation is not simply maximised everywhere.

Skin needs some ultraviolet exposure for vitamin D synthesis. Heavy pigmentation reduces that, which at high latitudes with weak winter sun becomes a real cost. The global pattern of skin pigmentation tracks latitude reasonably well, and the usual explanation is exactly this balance — enough protection where the sun is strong, enough transmission where it is weak.

That is a nice illustration of the general point: evolution rarely produces maximal anything. It produces compromises tuned to a particular environment, and moving the population moves them off their compromise.

Worth noting the other defences that are easy to overlook, because they are behavioural rather than anatomical.

Seeking shade in the middle of the day, covering up, and simply disliking the feeling of burning are all real protections, and every human society in a sunny climate developed clothing and shade practices. Discomfort is itself an adaptation — it is the signal that stops you before the damage gets worse.

Modern life removes a lot of that. Sunburn on a beach holiday takes hours to hurt, by which time it has happened.